Title: The Discovery of Color A Personal Perspective
1The Discovery of ColorA Personal Perspective
- O. W. Greenberg
- Miami 2007
- December 13, 2007
2Outline
- Developments in particle physics prior to the
work on color. - Discovery of color as a quantum number.
- Introduction of gauged SU(3) color.
3Particle physics prior to color
- The muon and pion had been discovered.
- Strange particles were found in cosmic rays.
- Lambda and Sigma hyperons.
- Kaon and antikaon, both charged and neutral.
- Xi, the cascade the Omega minus.
- Tau-theta puzzle.
4Accelerators come online
- About 1½ V events per day in a bubble chamber on
a medium-height mountain. - Separated beams of 106 Ks every 3 sec. at the
AGS - New problem to avoid swamping the detectors.
- Major problem at the LHC.
5Paradox copious production, slow decay.
- Attempt to understand using known dynamics
- Potential barriers, possibly connected with spin
could inhibit decaysdid not work.
6Paradox copious production, slow decay,
(continued).
- A. Pais, associated production.
- Strangeness is conserved for rapid production by
strong and electromagnetic interactions - Violated for slow decay by weak interactions.
7Strangeness
- Gell-Mann, Nakano and Nishijimadisplaced charge
multiplets. - Nishijima, Gell-Mann formula, QI3Y/2.
- Weak interaction selection rules.
8K-zero, K-zero bar complex
- K1, K2 with different decay modes, lifetimes.
- Particle mixing effects, regeneration.
- Beautiful illustration of superposition principle
of quantum theory.
9Tau-theta puzzle
- Tau?3 pi
- Theta?2 pi
- Same lifetimes
- Bruno Rossiprobably one particle
10Tau-theta puzzle, (continued)
- Dalitz analysis?different parities
- Parity was considered sacred
- The plot thickens
- The unexpected stimulates thought
11Tau-theta puzzle (continued)
- Suggestions by Lee and Yang
- Possible Interference Phenomena between Parity
Doublets - Question of Parity Conservation in Weak
Interactions, 22 June 1956
12Tau-theta puzzle, (continued)
- Lee and Yang proposed parity doublets to explain
this puzzle. - Lee and Yang examined the data for conservation
of parity, and found there was no evidence for
parity conservation in weak interactions. - Two solutions for one problemcant both be
correct.
13Wigners comment
- Why should parity be violated before the rest of
the Lorentz group? - Why is that surprising?
- Discrete transformations are independent of the
connected component of the Lorentz group.
14Parity violation was found earlier?
- Double scattering of beta decay electrons,
- R.T. Cox, et al., PNAS 14, 544 (1928).
- Redone with electrons from an electron gun with
much higher statistics. No effect seen, - C.J. Davisson and L.H. Germer,
- Phys. Rev. 33, 760 (1929).
15 Wightman, Axiomatic Quantum Field Theory
- Asymptotic condition in quantum field
theoryformalization of LSZ scattering theory. - Purely theoreticalno numbers, except to label
pages and equations. - Operator-valued distributions, relative
mathematical rigor.
16Divergent influences
- Very simple ideas used to classify newly
discovered particles. - Sophisticated techniques based on quantum field
theory.
17Interest in identical particles
- Why only bosons or fermions?
- Are there other possibilities?
- H.S. Greens parastatistics (1953) as a
generalization of each type. - Bosonparaboson, order p,
- Fermionparafermion, order p
- p1 is Bose or Fermi.
181962 Naples, Istanbul, SACLAY
- Axiomatic version of parastatistics with
DellAntonio and Sudarshan in Naples. - Presented at NATO summer school in Bebek, near
Istanbul. - Starting a collaboration with Messiah after
giving a talk at SACLAY.
19Istanbul
- NATO summer school organized by Feza Gursey at
the Robert College in Bebek - Eduardo Caianiello, Sidney Coleman, David
Fairlie, Shelly Glashow, Arthur Jaffe, Bruria
Kauffman, Louis Michel, Giulio Racah, Eugene
Wigner
20SACLAY with Messiah
- Albert Messiah, who fought with the Free French
army of General Leclerc, was at SACLAY - Entering SACLAY with guards on either side.
21Generalized statistics
- First quantized theory that allows all
representations of the symmetric group. - Theorems that show the generality of
parastatisticsGreens ansatz is not necessary.
221964
- Crucial year for the discovery of quarks and
color.
23The crucial year, 1964
- Gell-Mannquarkscurrent quarks.
- Zweigacesconstituent quarks.
- Why only qqq and q-qbar?
- No reason in the original models.
24Background, Princeton, Fall 1964
- Relativistic SU(6), Gursey and Radicati
- Generalization of Wigners nonrelativistic
nuclear physics idea to combine SU(2)I with
SU(2)S to get an SU(4) to classify nuclear
states. - Gursey and Radicati combined SU(3)f with SU(2)S
to get an SU(6) to classify particle states.
25SU(6) classifications
26Mesons
27Baryons
28Statistics paradox
29Attempts to make a higher dimensional
relativistic theory
- U(6,6)
- U(12)
- GL(12,C)
- Pais, Salam, et al, Freund, et al.
- Pais, Rev. Mod. Physics 38, 215 (1966).
30Magnetic moment ratio
31(No Transcript)
32Previous calculations of magnetic moments
- Complicated calculations using pion clouds
failed. - Nobody even realized that the ratio was so simple.
33Significance of the magnetic moment calculation
- A simple one-line calculation gave the ratio
accurate to 3. - Very convincing additional argument for the quark
model. - Quarks have concrete reality.
34The spin-statistics theorem
- Particles that have integer spin
- must obey Bose statistics
- Particles that have odd-half-integer spin must
obey Fermi statistics.
35Generalized spin-statistics theorem
- Not part of general knowledge
- Particles that have integer spin must obey
parabose statistics and particles that have
odd-half-integer spin must obey parafermi
statistics. - Each family is labeled by an integer p p1 is
ordinary Bose or Fermi statistics.
36Parafermi quark model, 1964
- Suggested a model in which quarks carry order-3
parafermi statistics. - This allows up to three quarks in the same
space-spin-flavor state without violating the
Pauli principle, so the statistics paradox is
resolved. - This leads to a model for baryons that is now
accepted.
37Resolution of the statistics paradox
- Exhilaratedresolving the statistics problem
seemed of lasting value. - Not interested in higher relativistic groups
from ORaifeartaighs and my own work I knew that
combining internal and spacetime symmetries is
difficult or impossible..
38No-go theorems
- Later work of Coleman and Mandula and of Haag,
Lopuszanski and Sohnius showed that the only way
to combine internal and spacetime symmetries in a
larger group is supersymmetry.
39Baryon spectroscopy
- Hidden parafermi (color) degree of freedom takes
care of the required antisymmetry of the Pauli
principle. - Quarks can be treated as Bosons in the visible
space, spin and flavor degrees of freedom.
40Table of excited baryons
- Developed a simple bound state model with s and p
state quarks in the 56, L0 and 70, L1
supermultiplets.
41(No Transcript)
42Later developments of baryon spectroscopy
- OWG, Resnikoff
- Dalitz, and collaborators
- Isgur and Karl
- Riska and collaborators
43How did the physics community react?
- J. Robert Oppenheimer
- Steven Weinberg
44Gave Oppenheimer a preprint in Princeton
- Met him at a conference in Maryland
- Greenberg, its beautiful!
45Oppenheimers response, (continued)
- but I dont believe a word of it.
46Weinberg, The making of the standard model
- At that time I did not have any faith in the
existence of quarks. (1967)
47Sources of skepticism
- Quarks had just been suggested.
- Fractional electric charges had never been seen.
- Gell-Mann himself was ambiguous.
48Gell-Manns comments
- It is fun to speculate if they were physical
particles of finite mass (instead of purely
mathematical entities as they would be in the
limit of infinite mass).A search would help to
reassure us of the non-existence of real quarks. -
49Skepticism, continued
- Assuming a hidden degree of freedom on top of the
fractionally charged unseen quarks seemed to
stretch credibility to the breaking point. - Some felt that parastatistics was inconsistent.
50Other solutions to the statistics paradox
- Explicit color SU(3), Han-Nambu, 1965
- Complicated antisymmetric ground state
- Quarks are not real anyway
- Other models
51Other solutions
- Explicit color SU(3)Han-Nambu, 1965
- Used three dissimilar triplets in order to have
integer charges. - Introduce now eight gauge vector fields which
behave as (1,8), namely as an octet in SU(3)''.
52Color electromagnetism commute
- Identical fractional electric charges allow color
electromagnetism to commute. - Allows color to be an exact, unbroken, symmetry.
- Crucial part of understanding of quantum
chromodynamics, QCD.
53Attempt to avoid a new degree of freedom
- Dalitz preferred a complicated ground state that
would avoid the statistics problem. - As rapporteur Dalitz always put a model with
Fermi quarks first. - The first rapporteur who preferred the
parastatistics model was Harari, Vienna, 1968.
54Arguments for a simple ground state
- General theorems lead to an s-wave ground state.
- The simplest antisymmetric polynomial in the
quark coordinates is
55Arguments for a simple ground state (continued)
- Then not clear what to choose for excited
states. - The polynomial
- vanishes because the coordinates are
linearly dependent. - Adding pairs leads to unseen exploding
SU(3) states that are not seen.
56Arguments for a simple ground state (continued)
- Zeroes in the ground state wave function
would lead to - zeroes in the proton electric and magnetic form
factors, which are not seen.
57If quarks are not real?
- If quarks are just mathematical constructs, then
their statistics is irrelevant.
58Other models
- Baryonettes, in which 9 objects (baryonettes)
compose a hadron. - Many other models.
59Saturation
- Why are the hadrons made from just two
combinations,
60Work with Zwanziger, 1966
- We surveyed existing models and constructed new
models to account for saturation. - The only models that worked were the parafermi
model, order 3, and the equivalent three-triplet
or color SU(3) models.
61Equivalence as a classification symmetry
-
- States that are bosons or fermions in the
parafermi model, order 3, - are in
- one-to-one correspondence with the states that
are color singlets in the SU(3) color model.
62Relations and differences between the models
63Properties that require gauge theory
- Confinement
- S. Weinberg, 1973
- D.J. Gross and F. Wilczek, 1973
- H. Fritzsch, M. Gell-Mann and H. Leutwyler, 1973
64Properties that require gauge theory (continued)
- Asymptotic freedom,
Gross, Wilczek, 1973 - Politzer, 1973
- Reconciles quasi-free quarks of the parton model
with confined quarks of low-energy hadrons
65Properties that require gauge theory (continued)
- Running of coupling constants and precision tests
of QCD. - Jets in high-energy collisions.
66Summary of introduction of color
67Two facets of strong interaction
- Color as a classification symmetry and a global
quantum number - parafermi model (1964)
- was the first introduction of color as a global
quantum number. - SU(3) color as a local gauge theory
- Han-Nambu model (1965) was the first
introduction of gauged SU(3) color.
68Outstanding puzzles of the standard model
- The reason for three generations of quarks and
leptons, - Origin of quark masses and the CKM matrix,
- Does the Higgs exist, if not what replaces it?
69Conclusion
- I reviewed the discovery of color, one of the
properties of the standard model. - Not clear where the next advances will come
fromsimple physical ideas or deep mathematically
motivated concepts. Time, and new experimental
data, will tell.